This invention relates to mechanisms for enabling the operator of a vehicle to adjust engine speed, transmission drive ratio and drive direction with a single control element and more particularly to control of a vehicle having a hydrostatic transmission and in which vehicle speed is varied in part by adjusting drive ratio and in part by adjusting engine speed.
Certain vehicles, of which industrial lift trucks and loaders are typical examples, are often equipped with a hydrostatic transmission which provides for infinite variation of the drive ratio between the engine and drive wheels or the like in both forward and reverse drive directions. The drive ratio and the drive direction are controllable by selectively moving a control member away from a central or neutral position. Motion in one direction progressively increases drive ratio in the forward drive mode while motion of the control member in the opposite direction from the neutral position progressively increases drive ratio in the reverse drive mode. The operator's task is greatly simplified in that shifting of a transmission is unnecessary and vehicle speed, drive ratio and drive direction may all be controlled through movement of a single member.
Under most conditions it is desirable that engine speed be increased as drive ratio increases in either forward or reverse directions and preferably this engine control function should also be accomplished by the same movement of the operator's control member which adjusts the transmission. However, the movement of the engine speed control element away from the idling position that is required for this purpose must be in the same direction regardless of whether the operator's control member is moved forward or backward.
Interconnections between the operator's control member and the transmission and the engine are still further complicated as it is desirable that the operator be able to further increase engine speed after the transmission drive ratio control element has reached its maximum movement in either the forward or reverse direction. This may be desirable, for example, when the vehicle is travelling relatively fast at the maximum available drive ratio and either a further speed increase is desired or the vehicle is subjected to increased loading such as may occur from starting up an inclined slope. It is then necessary to further open the engine throttle, governor rack or the like in order to maintain speed.
Thus, if the operator is to control both the hydrostatic transmission and engine speed with a single control member, the linkage which interconnects these components must be capable of shifting the transmission control element in either direction from a neutral position in accordance with the corresponding movement of the operator's control member and must concurrently shift the engine speed control a proportionate amount but always in the same direction regardless of the direction of movement of the control member. Further, the linkage must enable continued movement of the engine speed control in a single direction after the transmission control has reached the limit of travel in either direction.
Because of these complicated requirements, the linkages heretofore used for interconnecting a single operator's control member with both a hydrostatic transmission and an engine have been undesirably complex and costly or else have been limited with respect to providing maximum flexibility of the several desired control functions.